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Hubert Klahr - One of the best experts on this subject based on the ideXlab platform.
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convective overstability in radially stratified Accretion Disks under thermal relaxation
The Astrophysical Journal, 2014Co-Authors: Hubert Klahr, Alexander HubbardAbstract:This paper expands the stability criterion for radially stratified, vertically unstratified Accretion Disks incorporating thermal relaxation. We find a linear amplification of epicyclic oscillations in these Disks that depends on the effective cooling time, i.e., an overstability. The growth rates of the overstability vanish for both extreme cases, e.g., infinite cooling time and instantaneous cooling, i.e., the adiabatic and fully isothermal cases. However, for thermal relaxation times τ on the order of the orbital frequency, τΩ ~ 1, modes grow at a rate proportional to the square of the Brunt-Vaisala frequency. The overstability is based on epicyclic motions, with the thermal relaxation causing gas to heat while radially displaced inward and cool while radially displaced outward. This causes the gas to have a lower density when moving outward compared to when it moves inward, so it feels the outward-directed pressure force more strongly on that leg of the journey. We suggest the term "convective overstability" for the phenomenon which has already been studied numerically in the nonlinear regime in the context of amplifying vortices in Disks under the name "subcritical baroclinic instability." The aim of the present paper is to make clear that vortex formation in three-dimensional Disks is not necessarily subcritical, i.e., does not need a finite perturbation, nor is it baroclinic in the sense of geophysical fluid dynamics, which requires on vertical shear. We find that convective overstability is a linear instability that will operate under a wide range of physical conditions for circumstellar Disks.
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convective overstability in radially stratified Accretion Disks under thermal relaxation
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: Hubert Klahr, Alexander HubbardAbstract:This letter expands the stability criterion for radially stratified, vertically {unstratified} Accretion Disks incorporating thermal relaxation. We find a linear amplification of epicyclic oscillations in these Disks that depends on the effective cooling time, i.e. an overstability. The growth rates of the overstability vanish for both extreme cases, e.g. infinite cooling time and instantaneous cooling, i.e. the adiabatic and fully isothermal cases. However, for thermal relaxation times $\tau$ on the order of the orbital frequency, $\tau\Omega \sim 1$, modes grow at a rate proportional to the square of the Brunt-V\"ais\"al\"a frequency. The overstability is based on epicyclic motions, with the thermal relaxation causing gas to heat while radially displaced inwards, and cool while radially displaced outwards. This causes the gas to have a lower density when moving outwards compared to when it moves inwards, so it feels the outwards directed pressure force more strongly on that leg of the journey. We suggest the term ``Convective Overstability" for the phenomenon that has already been numerically studied in the non-linear regime in the context of amplifying vortices in Disks, under the name ``Subcritical Baroclinic Instability". The point of the present paper is to make clear that vortex formation in three-dimensional Disks is neither subcritical, i.e. does not need a finite perturbation, nor is it baroclinic in the sense of geophysical fluid dynamics, which requires on vertical shear. We find that Convective Overstability is a linear instability that will operate under a wide range of physical conditions for circumstellar Disks.
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the global baroclinic instability in Accretion Disks ii local linear analysis
The Astrophysical Journal, 2004Co-Authors: Hubert KlahrAbstract:This paper contains a local linear stability analysis for Accretion Disks under the influence of a global radial entropy gradient β = -d log T/d log r for constant surface density. Numerical simulations suggested the existence of an instability in two- and three-dimensional models of the solar nebula. The present paper tries to clarify, quantify, and explain such a global baroclinic instability for two-dimensional flat Accretion disk models. As a result, linear theory predicts a transient linear instability that will amplify perturbations only for a limited time or up to a certain finite amplification. This can be understood as a result of the growth time of the instability being longer than the shear time, which destroys the modes that are able to grow. Thus, only nonlinear effects can lead to a relevant amplification. Nevertheless, a lower limit on the entropy gradient ∝β ≈ 0.22 for the transient linear instability is derived, which can be tested in future nonlinear simulations. This would help to explain the observed instability in numerical simulations as an ultimate result of the transient linear instability, i.e., the global baroclinic instability.
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turbulence in Accretion Disks vorticity generation and angular momentum transport via the global baroclinic instability
The Astrophysical Journal, 2003Co-Authors: Hubert Klahr, Peter BodenheimerAbstract:In this paper we present the global baroclinic instability as a source for vigorous turbulence leading to angular momentum transport in Keplerian Accretion Disks. We show by analytical considerations and three-dimensional radiation-hydrodynamic simulations that, in particular, protoplanetary Disks have a negative radial entropy gradient, which makes them baroclinic. Two-dimensional numerical simulations show that a baroclinic flow is unstable and produces turbulence. These findings are tested for numerical effects by performing a simulation with a barotropic initial condition, which shows that imposed turbulence rapidly decays. The turbulence in baroclinic Disks transports angular momentum outward and creates a radially inward-bound Accretion of matter. Potential energy is released, and excess kinetic energy is dissipated. Finally, the reheating of the gas supports the radial entropy gradient, forming a self-consistent process. We measure Accretion rates in our two-dimensional and three-dimensional simulations of = -10-9 to -10-7 M? yr-1 and viscosity parameters of ? = 10-4 to 10-2, which fit perfectly together and agree reasonably with observations. The turbulence creates pressure waves, Rossby waves, and vortices in the (R, )-plane of the disk. We demonstrate in a global simulation that these vortices tend to form out of little background noise and to be long-lasting features, which have already been suggested to lead to the formation of planets.
Ryuichi Matsuba - One of the best experts on this subject based on the ideXlab platform.
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p process nucleosynthesis inside supernova driven supercritical Accretion Disks
The Astrophysical Journal, 2003Co-Authors: Shin Ichirou Fujimoto, Masaaki Hashimoto, Osamu Koike, Kenzo Arai, Ryuichi MatsubaAbstract:We investigate p-process nucleosynthesis in a supercritical Accretion disk around a compact object of 1.4 M☉, using the self-similar solution of an optically thick advection-dominated flow. Supercritical Accretion is expected to occur in a supernova with fallback material accreting onto a newborn compact object. It is found that an appreciable number of p-nuclei are synthesized via the p-process in supernova-driven supercritical Accretion Disks (SSADs) when the Accretion rate = c2/(16LEdd) > 105, where LEdd is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSADs have features similar to those of the oxygen/neon layers in Type II supernovae when the abundance of the fallback gas far from the compact object is that of the oxygen/neon layers in the progenitor. The overall abundance profile is in agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and La, are underproduced in the SSADs as in Type II supernovae. If the fallback gas is mixed with a small fraction of protons through Rayleigh-Taylor instability during the explosion, significant amounts of 92Mo are produced inside the SSADs. Isotopes 96Ru and 138La are also produced when the fallback gas contains abundant protons, although the overall abundance profile of p-nuclei is rather different from that of the solar system. The p-process nucleosynthesis in SSADs contributes to the chemical evolution of p-nuclei, in particular 92Mo, if several percent of the fallback matter are ejected via jets and/or winds.
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p process nucleosynthesis inside supernova driven supercritical Accretion Disks
arXiv: Astrophysics, 2002Co-Authors: Shin Ichirou Fujimoto, Masaaki Hashimoto, Osamu Koike, Kenzo Arai, Ryuichi MatsubaAbstract:We investigate p-process nucleosynthesis in a supercritical Accretion disk around a compact object of 1.4 M_solar, using the self-similar solution of an optically thick advection dominated flow. Supercritical Accretion is expected to occur in a supernova with fallback material accreting onto a new-born compact object. It is found that appreciable amounts of p-nuclei are synthesized via the p-process in supernova-driven supercritical Accretion Disks (SSADs) when the Accretion rate m_dot = M_dot c^2/(16 L_Edd) >10^5, where L_Edd is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSADs have similar feature to those of the oxygen/neon layers in Type II supernovae when the abundance of the fallback gas far from the compact object is that of the oxygen/neon layers in the progenitor. The overall abundance profile is in agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and La, are underproduced in the SSADs as in Type II supernovae. If the fallback gas is mixed with a small fraction of proton through Rayleigh-Taylor instability during the explosion, significant amounts of Mo92 are produced inside the SSADs. Ru96 and La138 are also produced when the fallback gas contains abundant proton though the overall abundance profile of p-nuclei is rather different from that of the solar system. The p-process nucleosynthesis in SSADs contributes to chemical evolution of p-nuclei, in particular Mo92, if several percents of fallback matter are ejected via jets and/or winds.
Shin Ichirou Fujimoto - One of the best experts on this subject based on the ideXlab platform.
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p process nucleosynthesis inside supernova driven supercritical Accretion Disks
The Astrophysical Journal, 2003Co-Authors: Shin Ichirou Fujimoto, Masaaki Hashimoto, Osamu Koike, Kenzo Arai, Ryuichi MatsubaAbstract:We investigate p-process nucleosynthesis in a supercritical Accretion disk around a compact object of 1.4 M☉, using the self-similar solution of an optically thick advection-dominated flow. Supercritical Accretion is expected to occur in a supernova with fallback material accreting onto a newborn compact object. It is found that an appreciable number of p-nuclei are synthesized via the p-process in supernova-driven supercritical Accretion Disks (SSADs) when the Accretion rate = c2/(16LEdd) > 105, where LEdd is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSADs have features similar to those of the oxygen/neon layers in Type II supernovae when the abundance of the fallback gas far from the compact object is that of the oxygen/neon layers in the progenitor. The overall abundance profile is in agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and La, are underproduced in the SSADs as in Type II supernovae. If the fallback gas is mixed with a small fraction of protons through Rayleigh-Taylor instability during the explosion, significant amounts of 92Mo are produced inside the SSADs. Isotopes 96Ru and 138La are also produced when the fallback gas contains abundant protons, although the overall abundance profile of p-nuclei is rather different from that of the solar system. The p-process nucleosynthesis in SSADs contributes to the chemical evolution of p-nuclei, in particular 92Mo, if several percent of the fallback matter are ejected via jets and/or winds.
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p process nucleosynthesis inside supernova driven supercritical Accretion Disks
arXiv: Astrophysics, 2002Co-Authors: Shin Ichirou Fujimoto, Masaaki Hashimoto, Osamu Koike, Kenzo Arai, Ryuichi MatsubaAbstract:We investigate p-process nucleosynthesis in a supercritical Accretion disk around a compact object of 1.4 M_solar, using the self-similar solution of an optically thick advection dominated flow. Supercritical Accretion is expected to occur in a supernova with fallback material accreting onto a new-born compact object. It is found that appreciable amounts of p-nuclei are synthesized via the p-process in supernova-driven supercritical Accretion Disks (SSADs) when the Accretion rate m_dot = M_dot c^2/(16 L_Edd) >10^5, where L_Edd is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSADs have similar feature to those of the oxygen/neon layers in Type II supernovae when the abundance of the fallback gas far from the compact object is that of the oxygen/neon layers in the progenitor. The overall abundance profile is in agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and La, are underproduced in the SSADs as in Type II supernovae. If the fallback gas is mixed with a small fraction of proton through Rayleigh-Taylor instability during the explosion, significant amounts of Mo92 are produced inside the SSADs. Ru96 and La138 are also produced when the fallback gas contains abundant proton though the overall abundance profile of p-nuclei is rather different from that of the solar system. The p-process nucleosynthesis in SSADs contributes to chemical evolution of p-nuclei, in particular Mo92, if several percents of fallback matter are ejected via jets and/or winds.
Michel Tagger - One of the best experts on this subject based on the ideXlab platform.
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rossby wave instability and three dimensional vortices in Accretion Disks
Astronomy and Astrophysics, 2010Co-Authors: Heloise Meheut, Francine Casse, P Varniere, Michel TaggerAbstract:Context. The formation of vortices in Accretion Disks is of high interest in various astrophysical contexts, in particular for planet formation or in the Disks of compact objects. But despite numerous attempts it has thus far not been possible to produce strong vortices in fully three-dimensional simulations of Disks. Aims. The aim of this paper is to present the first 3D simulation of a strong vortex, established across the vertically stratified structure of a disk by the Rossby wave instability. Methods. Using the ersatile advection code (VAC), we set up a fully 3D cylindrical stratified disk potentially prone to the Rossby wave instability. Results. The simulation confirms the basic expectations obtained from previous 2D analytic and numerical works. The simulation exhibits a strong vortex that grows rapidly and saturates at a finite amplitude. On the other hand the third dimension shows unexpected additional behaviors that could be of strong importance in the astrophysical roles that these vortices can play.
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rossby wave instability and three dimensional vortices in Accretion Disks
arXiv: Solar and Stellar Astrophysics, 2010Co-Authors: Heloise Meheut, Francine Casse, P Varniere, Michel TaggerAbstract:Context. The formation of vortices in Accretion Disks is of high interest in various astrophysical contexts, in particular for planet formation or in the Disks of compact objects. But despite numerous attempts it has thus far not been possible to produce strong vortices in fully three-dimensional simulations of Disks. Aims. The aim of this paper is to present the first 3D simulation of a strong vortex, established across the vertically stratified structure of a disk by the Rossby Wave Instability. Methods. Using the Versatile Advection Code (VAC), we set up a fully 3D cylindrical stratified disk potentially prone to the Rossby Wave Instability. Results. The simulation confirms the basic expectations obtained from previous 2D analytic and numerical works. The simulation exhibits a strong vortex that grows rapidly and saturates at a finite amplitude. On the other hand the third dimension shows unexpected additional behaviours that could be of strong importance in the astrophysical roles that such vortices can play.
Natalia Shabaltas - One of the best experts on this subject based on the ideXlab platform.
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dissipation and vertical energy transport in radiation dominated Accretion Disks
The Astrophysical Journal, 2011Co-Authors: Omer Blaes, Julian H Krolik, Shigenobu Hirose, Natalia ShabaltasAbstract:Standard models of radiation-supported Accretion Disks generally assume that diffusive radiation flux is solely responsible for vertical heat transport. This requires that heat must be generated at a critical rate per unit volume if the disk is to be in hydrostatic and thermal equilibrium. This raises the question of how heat is generated and how energy is transported in MHD turbulence. By analysis of a number of radiation/MHD stratified shearing-box simulations, we show that the divergence of the diffusive radiation flux is indeed capped at the critical rate, but deep inside the disk, substantial vertical energy flux is also carried by advection of radiation. Work done by radiation pressure is a significant part of the energy budget, and much of this work is dissipated later through damping by radiative diffusion. We show how this damping can be measured in the simulations and identify its physical origins. Radiative damping accounts for as much as tens of percent of the total dissipation and is the only realistic physical mechanism for dissipation of turbulence that can actually be resolved in numerical simulations of Accretion Disks. Buoyancy associated with dynamo-driven, highly magnetized, nearly isobaric nonlinear slow magnetosonic fluctuations is responsible for the radiation advection flux and also explains the persistent periodic magnetic upwelling seen at all values of the radiation to gas pressure ratio. The intimate connection between radiation advection and magnetic buoyancy is the first example we know of in astrophysics in which a dynamo has direct impact on the global energetics of a system.